Eke I.,OncoRay National Center for Radiation Research in Oncology | Dickreuter E.,OncoRay National Center for Radiation Research in Oncology | Cordes N.,OncoRay National Center for Radiation Research in Oncology | Cordes N.,National Center for Radiation Research And TechnologyRadiotherapy and Oncology | Year: 2012

In ionization chambers, not all released charge is collected due to the recombination of charge carriers. This effect is taken into account by the saturation correction factor kS. A physical description of the correction factor has been established for pulsed radiation. However, it is only accurate when the pulse length is short compared with the collection time of the ionization chamber. In this paper we develop a description of the saturation correction for radiation pulses of arbitrary length. For this, a system of partial differential equations is solved iteratively. The numerical solutions are verified experimentally for a Roos ionization chamber (PTW TM34001) exposed to a pulsed electron beam. The results of this iterative procedure describe the experimental data well. The calculations are also possible for beam structures which are experimentally hard to get and thereby contribute to a better understanding and correct description of the saturation correction at arbitrary pulse length. Among other things the pulse length dependent distributions of the charge carriers in the ionization chamber is calculated, inclusive of the transition to the conditions prevailing in the case of continuous irradiation. Furthermore is shown that the formula for kS established by Hochhuser and Balk is applicable even at arbitrary pulse length, if pulse duration dependent effective values are used for the parameters a and p. These effective values have been determined for the Roos chamber at pulse lengths up to 300 s.

Repetitive transcranial magnetic stimulation (rTMS) has been used worldwide to treat depression. However, the exact physiological effects are not well understood. Pathophysiology of depression involves crucial limbic structures (e.g. insula), and it is still not clear if these structures can be modulated through neurostimulation of surface regions (e.g. dorsolateral prefrontal cortex, DLPFC), and whether rTMS-induced excitatory/inhibitory transmission alterations relate to fronto-limbic connectivity changes. Therefore, we sought proof-of-concept for neuromodulation of insula via prefrontal intermittent theta-burst stimulation (iTBS), and how these effects relate to GABAergic and glutamatergic systems. In 27 healthy controls, we employed a single-blind crossover randomised-controlled trial comparing placebo and real iTBS using resting-state functional MRI and magnetic resonance spectroscopy. Granger causal analysis was seeded from right anterior insula (rAI) to locate individualized left DLPFC rTMS targets. Effective connectivity coefficients within rAI and DLPFC were calculated, and levels of GABA/Glx, GABA/Cr and Glx/Cr in DLPFC and anterior cingulate voxels were also measured. ITBS significantly dampened fronto-insular connectivity and reduced GABA/Glx in both voxels. GABA/Glx had a significant mediating effect on iTBS-induced changes in DLPFC-to-rAI connectivity. We demonstrate modulation of the rAI using targeted iTBS through alterations of excitatory/inhibitory interactions, which may underlie therapeutic effects of rTMS, offering promise for rTMS treatment optimization.

Gas-filled ionization chambers are the most important radiation detectors in radiotherapy. The collected charge at the electrodes does not represent the total released charge due to the unavoidable recombination processes. This needs to be considered for precise dose measurements. A quantitative description and correction of the recombination effects is established for two cases: continuous radiation exposure and pulsed radiation fields of single pulses with vanishing pulse duration. This work derives formulas for calculating the saturation correction for pulsed beams of nonvanishing pulse duration.Recursive formulas are derived describing the spatio-temporal development of the charge density distributions in plane-parallel ionization chambers starting at neglected recombination. Pulse duration dependent effective chamber parameters are calculated, by comparing the coefficients of a series expansion for small recombination effects. These parameters are used afterward in the known formula for the saturation correction factor for pulsed irradiation with increased recombination effects which was established with the assumption of vanishing pulse duration.The formulas should be valid for pulse durations shorter than half the collection time of the chamber. They allow calculating the saturation correction factor for pulsed beams of nonvanishing pulse duration and arbitrary pulse dose, if chamber, beam, and filling gas parameters are known. The filling gas parameters could be determined from experimental data. The calculation results are in good agreement with already published experimental and simulated results for a Roos chamber.The new formulas can be applied to determine the expected saturation correction for pulsed beams of different pulse duration and pulse dose including new beams at accelerators of new technologies. More experimental validation by using other chambers and an extension of the new formalism to pulse durations longer than half the collection time of the chamber is desired.